This site is not set up the way Steam is set up. If you bought something for $5, then you get it for $5. If I add a new version for $10, then you won't be able to get it, because. paid $5. Yes, it's weird. Is it fair? No. But there's nothing I can do. This is how this site works. The only solution I can offer you is to refund $5 and add $5 to buy the game for $10.
Steam works completely differently. There, if you bought a product - you get it forever. Why the developers of this site do not do so - the answer must be asked from them.